The history of the steam engine is one of long and continuous innovation, with the principal goal being the increase in efficiency of the engine in converting fuel to work. This efficiency was initially extremely low (approximately 1%), and gradually increased through the 19th Century to approximately 20% in large engines. Modern central power stations, using very high pressure steam, minimum exhaust temperatures, turbines having 50 or more stages of steam expansion, and a large amount of ancillary equipment approach 40% efficiency. There is nonetheless a need for small scale power generators and co-generators of heat and electricity that are able to burn waste biomass, as produced from crop processing in rural communities that are not served by commercial power services. It is important that such small scale generators be as efficient as possible to maximize the amount of electricity that can be produced from available biomass, as well as to minimize carbon dioxide emissions per kilowatt-hour generated.
Later efforts to improve the efficiency of the steam engine focused on increasing the pressure of the steam produced in the boiler, and reducing the duration of steam admission to the cylinder, in relation to the time required for a complete piston stroke, so that a large portion of the work done by the steam could be done by its expansion, and not simply by its displacement of the piston at boiler pressure. The expansion of steam causes a drop in its pressure and temperature, which required provisions to minimize the contact of hot boiler pressure steam with the cool exhaust steam and cooler surfaces of the engine. In addition, expansion of steam is accompanied by the condensation of a portion of the steam to water, which is extremely detrimental to the efficiency of the engine if the water remains in the cylinder. Some of the most successful steam engine designs were quite complex, having ingenious mechanisms to time the opening and closure of the steam inlet and exhaust valves, or having multiple cylinders of successively larger size, so that the high pressure, high temperature steam could be partially expanded in the small cylinder, before passing sequentially to the larger medium pressure cylinder, and finally to the largest high pressure cylinder. This design became virtually standard for marine applications, having the benefit of minimizing the heat loss from the high pressure to the low pressure steam, but also providing more equal and constant loads on the crankshaft.
One of the last innovations in reciprocating steam engine design, especially for stationery motive power, was the “Uniflow” engine, fully developed by the German inventor Stumpf in the first decade of the 20th century. The uniflow engine was both comparatively efficient, as well as very simple. It was the first engine to have only its steam inlet valves in the cylinder heads, the exhaust being accomplished by ports or openings in the wall of the cylinder, midway along the length of the cylinder. The uniflow engine could achieve a high degree of expansion of the steam in a single cylinder, because the heads were not cooled by contact with wet, low temperature, low pressure exhaust steam. The uniflow design was licensed to steam engine manufacturers worldwide, in particular Skinner in the United States.
Further development of the steam engine during the 20th century was arrested or severely limited by the development of the internal combustion engine for mobile applications, and the steam turbine for large scale electric power generation. The reciprocating steam engine nonetheless continued to play a very important role through the end of WWII, powering a majority of freighters and troop transport ships, as well as the majority of locomotives. An obvious advantage of the steam engine over the internal combustion engine is its ability to burn low cost solid fuels, including coal and biomass. Less well known advantages of the steam engine over the steam turbine for small scale applications include its much lower cost and its ability to operate efficiently at partial load. These advantages are relevant to the production of electricity from biomass fuels in rural communities in developing countries, as well as the cogeneration of heat and electricity from biomass fuels, where the heat load, being extremely variable in building heating and many other applications, dictates the amount of power that can be generated.